Filters, such as bandpass filters, have numerous applications in communications and electronics. For example, in wireless communications a given frequency band must accommodate many wireless users. To accommodate so many users, stringent bandpass filtering requirements must be achieved because of the crowded frequency allocations provided.
At present, wireless handsets use fixed-tuned bandpass filters (BPFs) to meet their filtering specifications. The design of such filters is complicated because they must achieve the lowest possible passband insertion loss (I.L.) while simultaneously achieving a specified large out-of-band rejection. As a specific example, consider full band PCS CDMA handsets using fixed bandwidth filters. The PCS transmit (TX) band should have no more than xe2x88x923.5 dB I.L. in-band (1850 to 1910 MHz in the U.S.) while having at least a 38.0 dB out-of-band rejection in the receive (RX) band (1930 to 1990 MHz range).
Further, this BPF must meet these specifications with a maximum constraint on height. A typical height constraint in present day handsets, for example, is 4.0 mm or less. To meet these demanding electrical requirements yet possess the smallest possible size and height, high order ( greater than 2nd order) fixed-tuned filters constructed from either individual coaxial resonator elements or monoblock structures are usually necessary. In addition, to satisfy out-of-band rejection specifications, a transmission zero is usually required, increasing I.L. at the band edge. Because of variations in ceramics and fabrication tolerances, vendors must individually adjust the characteristics of fixed-tuned filters during their manufacture, driving costs higher.
Moreover, if more than one frequency band were to be supported (e.g., supporting the PCS bands in the U.S., Korea, and India) multiple fixed-tuned BPFs would be necessary, requiring extra switches which introduces additional loss. This is true, even if the power amplifier and low noise amplifier used have sufficient bandwidth to operate over these multiple bands.
A tunable BPF would allow the use of one BPF over several bands, or of a lower order filter to cover a bandwidth wider than a required passband at any particular time. To provide the tunability in a tunable BPF, a component capable of providing a variable capacitance is typically used.
Several structures are presently used to implement a variable capacitor. For example, movable parallel plates have been used for many years as the tuner in home radios. However, such plates are far too bulky, noisy, and impractical for use in most modern applications.
Another alternative, the electronic varactor, is a semiconductor device that adjusts capacitance responsive to an applied voltage. Because the varactor is typically noisy and lossy, particularly in applications above 500 MHz, it is ineffective for high-frequency, low-loss applications where high performance is required.
Another alternative, a micro-electro-mechanical-system (MEMS) is a miniature switching device that may switch between capacitors responsive to an applied control signal. It, however, is costly, difficult to manufacture and of unproven reliability. In most cases, it provides discrete tuning, in that a system must select between a finite (and small) number of fixed capacitors.
Ferroelectric tunable capacitors are another alternative that has been attempted. Ferroelectric (f-e) materials are a class of materials, typically ceramic rare-earth oxides, whose prominent feature is that their dielectric constant (xcexa), and as a consequence, the electric permittivity (∈) changes in response to an applied slowly varying (DC or low frequency) electric field. The relationship of the dielectric constant (xcexa) and the electric permittivity (∈) of a material is given as follows:
∈=xcexa∈0
where ∈0 is the electric permittivity of a vacuum. At present, there are several hundred known materials that possess f-e properties. In a typical f-e material, one can obtain a range in xcexa by a factor of as much as approximately 3:1. The required DC voltage to generate such a change in xcexa depends on the dimensions of the f-e material over which a DC control voltage is applied. As a result of their variable dielectric constant, one can make tunable capacitors using f-e materials, because the capacitance of a capacitor depends on the dielectric constant of the dielectric proximate the capacitor conductors. Typically, a tunable f-e capacitor is realized as a parallel plate (overlay), interdigital (IDC), or a gap capacitor.
In known f-e variable capacitors, a layer of an appropriate f-e material, such as barium strontium titanate, BaxSr1xe2x88x92xTiO3 (BSTO) is disposed adjacent to one or both conductors of a capacitor. Depending upon the strength of the electric field applied to the f-e material and the intrinsic properties of the f-e material selected, the capacitance changes. Typically, below the Curie temperature, Tc, of the f-e film, the f-e material is in the ferroelectric state and will exhibit hysteresis in its response to a changing electric field. Above Tc, f-e material is in the paraelectric state and will not exhibit hysteresis. Thus, one generally picks an f-e material whose Tc is lower than the expected operating temperature so as to operate in the paraelectric state, avoiding the hysteresis effects of the ferroelectric state.
However, conventional f-e variable capacitors have proven to be too lossy for use in insertion-loss-sensitive applications such as handsets. Moreover, these devices often perform unpredictably, preventing optimal design, construction, and use of f-e tunable filters.
Accordingly, there is a need in the art for improved tunable f-e filters capable of providing a tuning range over a desired frequency range with low I.L. and high out-of-band rejections and methods for designing the same.
Fixed tuned bandpass filters must satisfy stringent size, insertion loss and out of band rejection, among other requirements. Tunable filters would be useful in replacing fixed tuned bandpass filters if they could meet these requirements. Lower order, or otherwise better, tunable filters might be used to tune over ranges requiring higher order fixed tuned filters. Or a single tunable filter could replace more than one fixed tuned filter. However, tunable filters require tunable components that have consistently shown themselves to be to high in insertion loss, unreliable, or possessing other prohibitive qualities.
It is desirable to provide a tunable bandpass filter that has superior insertion loss properties with respect to fixed-tuned bandpass filters yet still achieves required rejection performance and satisfies other requirements. It is therefore an object of this invention to provide tunable bandpass filters incorporating ferro-electric materials to tune the filters while maintaining a low insertion loss, meeting stringent out of band rejection requirements and satisfying other requirements. This is made possible by the advantageous design of capacitors and filters based on a correct recognition of the loss characteristics of the ferro-electric materials.
Another object of the invention is to provide a methodology for designing tunable bandpass filters. This methodology quantifies and minimizes loss mechanisms in tunable ferro-electric capacitors to select optimal structures for a tunable bandpass filter incorporating tunable ferro-electric capacitors.
The primary object of this process is to allow the user to design minimum loss BPF""s that meet or exceed all other electrical and mechanical specifications placed on a conventional fix-tuned BPF that it replaces. The meeting or exceeding of performance specifications is critical if a tunable BPF is to replace a fix-tuned BPF in practical applications.
Proper f-e film characterization, along with optimum tunable BPF design procedures are mandatory if one is to achieve minimum loss tunable BPF""s that simultaneously meet a stringent rejection specification.
In accordance with one embodiment of the invention, a method is provided for choosing a bandwidth and filter order for a tunable bandpass filter to satisfy an out-of-band rejection requirement and a passband insertion loss requirement. Given a topology for a ferro-electric capacitor, the method calculates the non-ferro-electric losses for the ferro-electric capacitor. Given a resonator having a first quality factor for coupling to the ferro-electric capacitor, the method determining the required ferro-electric loss of the f-e capacitor based upon the calculated non-ferro-electric losses and the first quality factor to achieve an insertion loss requirement for the tunable bandpass filter.
In accordance with another embodiment of the invention, a process is described by which a wide variety of f-e films can be efficiently and correctly characterized.
Further aspects and features of the invention are set forth in the following description together with the accompanying drawings.